The present invention relates to the fields of boring and of making excavation screens in the ground.
More precisely, the invention relates to an excavator machine comprising:                a suspended casing having a top end and a bottom end;        at least one cable extending above the casing, said cable being under tension and having a bottom end fastened to the top end of the casing; and        a cutter device arranged at the bottom end of the casing.        
Such an excavator machine may particularly, but not exclusively, be a rotary drum boring machine, also referred to as a hydraulic cutter. FR 2 211 027 describes such a machine. During the boring operation, the casing moves downwards progressively as the rotary drums dig the trench.
In another variant, such an excavator machine is a clamshell bucket, actuated by a mechanical or hydraulic mechanism.
For certain kinds of work, the trench may present a great depth, possibly reaching 100 meters (m) or even more. Furthermore, it is generally necessary for such a trench to present great accuracy in terms of verticality, in particular because the final work is the result of juxtaposing panels, e.g. molded walls or any other type of screen.
In particular because of irregularities in the soil in which the trench is to be made, there are major risks of the casing being deflected from its vertical path, and this risk increases with increasing boring depth.
There thus exists a real need to have systems making it possible to monitor verticality, or at least orientation, concerning the movements of the casing in the ground, by detecting any offsets from the desired path.
To solve that problem, EP 0 841 465 proposes a system of monitoring the verticality of a boring machine in which two cables of small section are fastened to the top end of the machine. The cables are kept under constant tension and pass through two fixed reference points arranged at the top end of the trench. By continuously measuring the lengths of the cables, and also the tilt angles at the ends of the cables that are fastened to the machine, it is possible to calculate the coordinates of the two cable fastener points.
Although that method gives satisfaction for boring depths of less than 100 m, it nevertheless is not sufficiently accurate for boring the greater depths.